Pulse generator for producing the harmonic equivalent of a sawtooth wave

Abstract

An electronic organ tone generating technique produces a harmonic equivalent of a sawtooth wave. In one embodiment square waves generated by a conventional tone generator are applied to a one shot multivibrator, the output of which is a series of narrow rectangular pulses having a flat harmonic spectrum with both even and odd harmonics present. The narrow rectangular pulses are applied to a 3db per octave filter to produce a waveform, the harmonic content of which is equivalent to that of a sawtooth wave. In this embodiment, the pulse width remains constant regardless of the frequency of the pulse train. In a second embodiment, the duty cycle at each octave when moving up the keyboard is doubled to compensate for amplitude falloff which may occur in the filtered signal.

Having described the invention, the embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. In an electronic musical instrument having means for selecting any one of a pluraltiy of different pitch musical tones to be produced, a system for producing said selected tone comprising: means for generating a narrow duty cycle rectangular pulse train having a frequency proportional to and a duty cycle varying directly with the pitch of the selected tone, said duty cycle being equal to or less than 50% to cause the harmonic spectrum of said pulse train to be substantially flat over the audible range.   
     
     
       2. The electronic musical instrument of claim 1 wherein said filter means comprises a 3 db per octave low pass filter. 
     
     
       3. The electronic musical instrument of claim 1 wherein said means for generating the pulse train comprises a series connected tone generator and one shot multivibrator. 
     
     
       4. In an electronic musical instrument having means for selecting any one of a plurality of different pitch musical tones to be produced, a system for producing said selected tone comprising: means for generating a plurality of pulse trains having frequencies different from each other by powers of two and proportional to the pitch of the selected tone, including a first pulse train having the highest frequency;   octave select means coupled to the pulse train generating means to pass selected ones of the generated pulse trains;   means for combining the pulse trains passed by the octave select means to produce a narrow duty cycle pulse train having a duty cycle equal to or less than 50% and varying directly with the octave of the selected tone;   single filter means for causing the harmonic spectrum of the pulse train to approximate the harmonic spectrum of a sawtooth wave; and,   means for converting the filtered pulse train to sound.   
     
     
       5. The electronic musical instrument of claim 4 wherein said means for generating a pluraltiy of pulse trains includes a divider means for dividing the frequency of said first pulse train by powers of two. 
     
     
       6. The electronic musical instrument of claim 4 wherein said means for combining the pulse trains includes a NOR gate. 
     
     
       7. The electronic musical instrument of claim 4 wherein the duty cycle is doubled at each higher octave. 
     
     
       8. The electronic musical instrument of claim 4 wherein the filter means is a 3 db per octave low pass filter. 
     
     
       9. The electronic musical instrument of claim 4 wherein said octave select means comprises an octave latch having a plurality of outputs corresponding to the octaves in which said selected tone may fall and a matrix of couplers connected to said latch outputs and to said pulse train generating means for selectively coupling said pulse trains to said pulse train combining means. 
     
     
       10. In an electronic musical instrument having means for selecting any one of a pluraltiy of different pitch musical tones to be produced, a system for producing said selected tones comprising: means for generating a plurality of pulse trains having frequencies different from each other by powers of two and proportional to the pitch of the selected tone;   octave select means coupled to the pulse train generating means to pass selected ones of the generated pulse trains;   divider means for dividing the lowest frequency pulse train passed by the octave select means to generate a further plurality of pulse trains;   means for combining the pulse trains passed by the octave select means and generated by the divider means to produce a narrow duty cycle rectangular pulse train, said duty cycle being equal to or less than 50%;   single filter means for causing the harmonic spectrum of the pulse train to approximate the harmonic spectrum of a sawtooth wave; and   means for converting the filtered pulse train to sound.   
     
     
       11. The electronic musical instrument of claim 10 wherein the divider means divides the frequency of the lowest frequency pulse train by powers of two. 
     
     
       12. The electronic musical instrument of claim 10 wherein the duty cycle of the rectangular pulse train is doubled at each octave. 
     
     
       13. The electronic musical instrument of claim 10 wherein the filter means is a 3 db per octave low pass filter. 
     
     
       14. A method for producing any one of a plurality of musical tones having a predetermined harmonic content, comprising the steps of: selecting the pitch of a musical tone to be produced;   generating a rectangular pulse train having a frequency proportional to and a duty cycle varying directly with the selected pitch, said duty cycle being equal to or less the 50% to cause the harmonic spectrum of the pulse train to be substantially flat over the audible range;   passing said pulse train through a filter having characteristics approximating the harmonic spectrum of a sawtooth wave; and   converting said filtered pulse train to sound.   
     
     
       15. The method of producing musical tones of claim 14 wherein the filter is arranged to reduce the harmonic spectrum of the pulse train 3 db per octave. 
     
     
       16. A method for producing any one of a plurality of musical tones having a predetermined harmonic content, comprising the steps of: selecting the pitch of a musical tone to be produced;   generating a narrow duty cycle rectangular pulse train having a frequency proportional to the selected pitch, with said duty cycle being equal to or less than 50%;   doubling the duty cycle of said pulse train at each higher octave;   passing said pulse train through a filter having a filter characteristic approximating the harmonic distribution of a sawtooth wave; and,   converting said filtered pulse train to sound.